Asthma is a chronic disease affecting the air passages in the lungs. There’s no single cause of asthma. However, experts believe that a combination of genetic and environmental factors can cause the condition or at least make a person sensitive to asthma triggers. Other factors can include:

Still, no one really knows why some people are affected by asthma and others aren’t. Allergies are often associated with asthma, but not all people with allergies have asthma. While asthma causes are not known, doctors have identified the main causes of asthma symptoms.

According to the World Health Organization (WHO), asthma affects about 235 million people worldwide. While asthma is prevalent in both developed and developing nations, WHO states that at least 80 percent of asthma-related deaths occur in developing countries. This is perhaps due to a combination of a lack of awareness and a lack of access to treatments.

If you have asthma, the lining of your airways is inflamed (swollen). This inflammation makes the air passages particularly sensitive to irritants and asthma triggers. Inflammation can also narrow the air passages and make it difficult for air to pass through the airways. As a result, you’ll find it hard to breathe in and out.

When the airways encounter certain asthma triggers, the muscles around the airways tighten. This causes the air passages to become even narrower and gives you a tight feeling in the chest, like a rope is being tightened around it. Mucus can get lodged in the narrowed airways, causing more breathing difficulties.

The triggers that cause the inflammation and airway constriction can vary in different people. When the airway comes into contact with one of many asthma triggers, it becomes inflamed, constricts, and fills with mucus. The lining of the airway may swell, causing the airway to narrow.

Allergies have long been suspected as being a possible cause of asthma. In these cases, the condition is referred to as allergic asthma. The things you’re allergic to can trigger allergic asthma symptoms. For example, if you have seasonal pollen allergies, then you might also experience asthma symptoms during this time.

There also appears to be a link between asthma and a preexisting risk of being allergic to multiple substances (atopy). According to the University of Illinois, between 20 and 40 percent of people have atopy. Still, it’s unclear how many go on to develop asthma.

Asthma is diagnosed with a physical exam as well as tests that measure lung function. Two lung function tests used to detect asthma are peak flow and spirometry tests.

A peak flow test works with a meter that measures your breathing, and the results are tracked over a designated amount of time. Asthma may be suspected if your peak flow readings are low.

A spirometry test also measures your breathing, but in a different way. This test helps estimate how much trouble you have breathing air out. This is done by having you breathe in deeply and then seeing how much and how fast you can breathe out.

If allergic asthma is suspected, you may also be tested for allergies. Blood testing is common with food allergies. For most other allergies, though, skin testing yields more accurate results. This works by pricking the skin and inserting a small amount of a suspected substance. After several minutes, your doctor will then see if your skin reacts. A positive reaction looks like a large, red bump.

Asthma continues to be a serious public health concern, especially among children. While fatalities aren’t uncommon in developing nations, the outcome tends to be much more positive in developed countries where resources and early detection are abundant.

Following an asthma diagnosis, your goal will be to maintain your condition and to help prevent asthma attacks. While not necessarily fatal, asthma attacks can lead to hospitalization from severe symptoms.

Last medically reviewed on May 25, 2017

Medically reviewed by Adithya Cattamanchi, MD — Written by the Healthline Editorial Team and Kristeen Cherney — Updated on August 15, 2018